Little Magnetic Discs Turn AA Batteries into Self-Propelled Motors

By attaching circular magnets to an AA battery, you can make a little homopolar motor that uses an electric current and magnetic field to propel itself across a conductive surface. It's one of the simplest motors around, and you can make it at home with supplies from the hardware store.

A homopolar motor is driven by the Lorentz force. When you place an electrified conductor in a magnetic field that runs perpendicular to it, it'll generate torque and move the conductor. In the case of the little rolling Duracell cars, the aluminum foil acts as the conductor, and the resulting torque moves the magnetic discs—which are free to roll—and propels the battery-cars along the conductive surface.

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The Lorentz force is also used to fire a projectile from a railgun and dictates the path of particles in circular particle accelerators. But at home, if you're not up to building a railgun, you can use this electromagnetic force to turn batteries into little DIY cars and race them across the countertop.

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